1. Field of the Invention
This invention relates to a projector using a hologram.
2. Related Background Art
As a prior-art projector, there is known one of a construction as shown in FIG. 1 of the accompanying drawings wherein a polarizing beam splitter 703 is disposed at the destination of emergence of white parallel light emitted from a light source unit 702 and further, first and second dichroic mirrors 704 and 705 formed on the cemented surface of 45.degree. rectangular prisms for separating S-polarized light reflected by the polarizing beam splitter 703 into red, green and blue lights and applying these color lights to first to third liquid crystal panels 701R, 701G, 701B of the reflection type which form image lights of respective colors and combining the image lights of respective colors which have emerged from the first to third liquid crystal panels 701R, 701G, 701B are successively arranged in the optical path of said S-polarized light and the image lights combined by the first and second dichroic mirrors 704 and 705 are projected toward a screen, not shown, through a projection lens 706 to thereby form an enlarged color image on the screen (see, for example, Japanese Laid-Open Patent Application No. 61-13885).
As the first to third liquid crystal panels 701R, 701G, 701B of the reflection type, use is made of those of electrically controlled birefringence (ECB) type, which have the characteristic of rotating the plane of polarization of incident S-polarized light by an applied voltage conforming to an image signal for each color.
In the liquid crystal type projector of the above-described construction, of the substantially parallel white light emitted from the light source unit 702, the S-polarized light is reflected by the polarizing beam splitter 703, and the S-polarized light is separated into red, green and blue lights by the first and second dichroic mirrors 704 and 705, and these color lights are applied to the first to three liquid crystal panels 701R, 701G, 701B corresponding to the respective color lights. The color lights reflected by the first to third liquid crystal panels 701R, 701G, 701B include a ray of P-polarized component having its plane of polarization rotated in conformity with an image signal input to each picture element of each panel and a ray of S-polarized component not subjected to the rotation of the plane of polarization, and those color lights are again combined by the first and second dichroic mirrors 704 and 705, and thereafter travel toward the polarizing beam splitter 703. The P-polarized component of each color light is transmitted through the polarizing beam splitter 703 and projected onto the screen, not shown, through the projection lens 706, and the S-polarized component of each color light is reflected by the polarizing beam splitter 703 and returns toward the light source unit 702.
However, the above-described prior-art projector suffers from the following problem. The light beam incident on the first and second dichroic mirrors for effecting the color separation of light and the combination of color lights has its direction of polarization varied before and after the reflection by the first to third liquid crystal panels for forming the image lights of respective colors, i.e., during the color separation by the first and second dichroic mirrors and during the color combination by these dichroic mirrors. In a dichroic mirrors and during the color combination by these dichroic mirrors. In a dichroic mirror of optical multilayer film formed on the cemented surface of 45.degree. rectangular prisms, like the above-described first and second dichroic mirrors, taking the light of blue wavelength range as an example, the reflection band width of S-polarized component is wide as compared with that of P-polarized component, as shown in FIG. 2 of the accompanying drawings, and reflection characteristic differs depending on the polarized components. Accordingly, when as in the aforedescribed liquid crystal projector shown in FIG. 1, the incident light onto the liquid crystal panel corresponding to each color is S-polarized light, the reflected light from that liquid crystal panel becomes P-polarized light and therefore, part of each color image light reflected from each liquid crystal panel is lost.